Thermally processable imaging elements, including films and papers, for producing images by thermal processing are well known. These elements include photothermographic elements in which an image is formed by imagewise exposure of the element to light followed by development by uniformly heating the element. These elements also include thermographic elements in which an image is formed by imagewise heating the element. Such elements are described in, for example, Research Disclosure, June 1978, Item No. 17029 and U.S. Pat. Nos. 3,080,254, 3,457,075 and 3,933,508.
One common photothermographic element comprises an oxidation-reduction image forming composition containing an organic silver salt oxidizing agent, a photocatalyst such as silver halide, and a reducing agent. The element is stable at ambient temperature, however, after exposure to light, the organic silver salt oxidizing agent and the reducing agent, when heated, undergo an oxidation-reduction reaction due to the catalytic action of the photocatalyst and form silver. The silver formed darkens the exposed areas of the photosensitive layer and creates an image.
A problem which frequently occurs with these types of elements is unintentional darkening of the unexposed areas of the photosensitive layer. This may be caused by reducing agent impurities in the coated element. This darkening is often referred to as fog. One compound which has been commonly used to suppress fog is mercuric ion. However it is known that mercury is detrimental to the environment.
Sulfenimide compounds containing an inhibitor fragment which is released during a conventional wet development process have been described in JP 04-194922, JP 04-186346, JP 04-157457, JP 04-350845, JP 63-163337 and DD 251 411. Phthalimide and various substituted phthalimides have been used widely as image tone modifiers and/or development accelerators in photothermographic or thermographic elements, for example, see U.S. Pat. No. 3,839,041. Phthalimides when substituted with electronegative groups, such as in 4-bromophthalimide or 4-cyanophthalimide, and incorporated into a photothermographic or thermographic film, have been described as reducing heat fog in U.S. Pat. No. 4,181,530. Certain N-substituted phthalimides (where N is a halogen atom) have been described as fog inhibitors in photothermographic films, U.S. Pat. No. 3,957,491. Finally, U.S. Pat. No. 5,439,790 describes the use of phthalimide blocked compounds of the general structure below for releasing a post processing stabilizer in a photothermographic film, where the A moiety can be, for example, a triazine or tetrazole group. ##STR2##
There is still, however, a continuing need in the industry for practical and environmentally benign stabilizers and fog preventers which do not otherwise adversely affect the performance of the thermographic element. The inventors herein have discovered that certain sulfenimide compounds are particularly useful as fog restrainers for such elements.